Process of treating metal and alloy articles to improve the resistivity thereof to corrosion and to the product thereof



June 5, 1934. 3 5 FlNK EI'AL 1,961,752

PROCESS OF TREATING METAL AND ALLOY ARTICLES TO IMPROVE THE RESISTIVITY THEREOF TO CORROSION AND TO THE PRODUCT THEREOF Filed July 23. 1931 AN E AG ITATOR Theamen!" 055 Patented June 5, 1934 UNITED STATES PATENT OFFICE PROCESS OF TREATING METAL AND ALLOY ARTICLES TO IMPROVE THE RESISTIVITY THEREOF TO CORROSION AND TO THE I PRODUCT THEREOF Colin G. Fink and Frederic J. Kenny, New York,

Application July 23, 1931, Serial No. 552,579 13 Claims. (01.204-1) The invention provides a process of treating metals (especially metals containing impurities) and alloy articles whereby the resistivity thereof to corrosion is markedly improved. Not only is the resistivity, as evaluated in terms of grams loss per unit area, greatly increased, but even more importantly the uniformity of the resistivity over the whole area is greatly improved.

The inventor further provides a metal or alloy product having improved resistivity as compared with an article of the same metal or alloy which has not been treated according to the present process.

The accompanying drawing shows diagrams and pictorial representations of assistance in describing the process and product of our invention.

Those metals and alloys which form or have formed thereon adherent oxid surfacefilms having an expansion characteristic nearly the same as that of the metal or alloy itself are, according to one accepted theory, those most resistant. to corrosion. If the surface or surface portion of these and other metals and alloys can be equipotentialized, the resistance thereof to corrosion should be increased. We have found that the surface or surface portion can be equipotentialized and that this can be accomplished by electrolytic treatment.

We have used the term equipotentializing in the specification and claims. By equipotentializing we mean a procedure by which the electropotential of two or more parts, constituents, areas or points become equalized.

The ideal condition or state when no longer any diiferences in potential remain, is not always attained, but according to our process this state is closely approached.

According to our invention therefore, we immerse the metal or alloy article to be treated in an electrolytic bath, connect it in the electric circuit, and dissolve by the aid of the electric current, a portion of its surface. The points or areas of high electrolytic solution potential will dissolve more rapidly or preferentially, and as the current is allowed to act a timeis reached when the points or areas of high anodic potential have been substantially eliminated, and the surface as a whole equipotentialized.

The electrolytic treatment is preferably carried out in a bath having an oxidizing action on the metal or alloy being electrolytically treated,

whereby at the same time that the surface, or'.

portions thereof, are being dissolved, there is also an oxidizing or passivating of the surface effected.

A mode of procedure in carrying out our process is as follows.

The alloy article subjected to treatment may be for example a commercial annealed sheet of chromium alloy or steel, or of the so-called 18-8 type, conforming to the following analysis:

Per cent Chromium 18.59 Nickel 8.28 Carbon 0.13 Manganese None Silicon 0.31 Sulphur 0.01 Phosphorus 0.06

Iron (except for minute traces of other elements) to make 100 A sheet of said chromium alloy X-is hung as an anode in a tank T containing a solution S of chromic acid, and current made to flow, thereby dissolving the surface or surface portions of the sheet X, reducing or substantially eliminating the points or areas of high anodic potential and equi potentializing the surface or surface portions of said sheet X. At the same time the surface of the sheet X is oxidized or passivated. After a suitable length of time in the bath subjected to the electrolytic treatment, the sheet X is removed, and preferably washed in water and dried.

The cathodes C, C may be of lead, or of the same 18-8 alloy or some other suitable material.

An agitator A is preferably provided for maintaining uniformity throughout the solution.

Means J, as a jacket through which water or steam, or both, may be circulated, are preferably provided for controlling the temperature of the solution S.

Considerable latitude may be exercised in the choice of concentration of the chromic acid solution, temperature of the bath and current density employed, and duration of treatment; the following conditions havebeen found to be most satisfactory:

Concentration of chromic acid 42.5% Temperature 40 C. Current density, amp eres per sq. cm .06 Time 1 hour Alloy and metal articles treated electrolytically in substantial accordance with the above indicated conditions, have the resistivity thereof to corrosion greatly increased. Also the uniformity of the resistivity of the alloy and metal articles to corrosion is greatly increased; that is, the tendency of treated alloy and metal articles having the increased resistivity is to corrode uniformly over the entire surface, and at the surface, whereas untreated alloy and metal articles corrode in spots, or locally, and also beneath the surface.

The character of the corrosion of two samples of 18-8 alloy, one untreated and shown in Fig. 2, and the other treated according to the present process, shown in Fig. 3, and both subjected to 100 hours corrosion in a 10% solution of ferric chloride is indicated to some extent in said figures. Much corrosion under the surface in Fig. 2 cannot be adequately shown pictorially. The difference in resistivity to corrosion is however indicated by the relative loss in weight between the samples, Figs. 2 and 3, the relation being The samples shown in Figs. 2 and 3 further illustrate samples of sheet-metal which have been strained by cutting and punching. Our invention eliminates the increased corrosive tendency due to such strains. This is clearly evident from a comparison of samples, Figs. 2 and 3. In Fig. 2, localized and pronounced corrosion is perceived around the edges and around the hole where the metal has been strained. This strain-corrosion of untreated samples around punched holes and cut edges is however better shown in Fig. 4 illustrating an actual sample.

This process of relieving strain corrosionhas the advantage that valuable mechanicalproperties such as tensile strength and hardness are not afiected by the treatment.

The average loss in weight per unit area of (a) an untreated piece of alloy; and (b) a piece of alloy treated electrolytically in chrom ic acid solution according to our invention (all from the same piece of alloy sheet) when subjected to corrosion in 10% ferric chlorid solution, have the relation of the number 11.8:1. That is to say, nickel-chromium alloy articles treated according to our invention are approximately 12 times as resistant to corrosion than the untreated alloy articles, on ;a weight loss basis. Samples subjected to atmospheric corrosion indicate a similar relation of corrosion resistivity. However, the character of the corrosion in treated and untreated samples is different, the corrosion of untreated samples being usually localized or pitted and beneath the surface, while the corrosion of the articles treated according to our invention is substantially uniform and at the surface.

Moreover, our tests have shown that corrosion along the grain boundaries (which makes the metal crumbly) is eliminated or materially diminished, when alloy and metal articles have been treated according to our invention disclosed herein. a

The ratio of the average deviation from the mean of electrolytic solution potentials measured on various points on the surface of untreated samples and of samples treated anodically as above described, has been found by us to be:--

average deviation for untreated samples average deviation for anodically treated samples While other electrolytic solutions may be used, best results have been obtained by the use of chromic acid solutions. Examples of other electrolytic solutions are, concentrated sulphuric acid solution, concentrated hydrochloric acid solution, concentrated nitric acid solution.

The graph, Fig. 5, shows the eifect of concentration of the chromic acid solution on the corrosion resistivity of treated samples of ironchromiurn-nickel alloy of the so-called 18-8 type tested for loss of weight after being subjected to corrosion in 10% ferric chloride solution. The graph indicates the desirability of using solutions containing 15% or more chromic acid.

The graph, Fig. 6, shows the effect of temperature of the chromic acid electrolytic bath on the resistivity of treated samples of iron-chromiumnickel alloy of the so-called 18-8 type tested for loss of weight, after being subjected to corrosion in 10% ferric chloride solution. The graph indicates the desirability of using chromic acid solutions at temperatures of 25 C. or thereabout', and above.

Appreciable increase in the resistivity of alloy and metal articles to corrosion have likewise been obtained by us by treating Monel metal, sheet iron, and brass articles to the process herein described.

The best results known to us obtained by the use of other processes for improving the resistivity of metaland alloy articles to corrosion, as compared with the resistivity of untreated metal and alloy articles, when subjected to equal corrosion treatment, measured by weight loss, is shown by the ratio of the number Moreover the character of the corrosion of samples treated according to such other processes is shown in Fig. 7. The pitting and intergranular corrosion is quite evident in the illustration.

The invention may be practiced by following other modes of procedure than that herein specifically described.

What is claimed is:-

1. A new article of metal containing impurities or alloy characterized by having a surface practically free from galvanic couples.

2. A new article of metal containing impurities or alloy having an equipotentialized surface.

3. A new article of metal containing impurities or alloy having an equipotentialized surface wherein the relation of the average deviation for untreated samples'to' the average deviation for anodically treated samples is approximately 4. A new article of metal containing impurities or alloy having an equipotentialized surface which when subjected to corrosion, corrodes uniformly substantially without pitting.

5. A new article of metal containing impurities or alloy having an equipotentialized surface in which corrosion along the grain boundaries is eliminated or materially diminished.

6. A iew article of metal containing impurities or alloy having an equipotentialized surface in which the weight loss by corrosion in ferric chloride solution as compared with an unequipotentialized sample is expressed by a lesser fraction than '7. A new acticle of metal containing impurities or alloy having an equipotentialized surface in which the weight loss by corrosion in ferric chloride solution as compared with an unequipotentialized sample is expressed by the fraction or a lesser fraction.

8. A'process of treating metal and alloy articles, in which differences in potential exist between difierent parts of the surface to increase the resistivity thereof to corrosion, comprising electro-chemically dissolving the high-potential portions of. the metal or alloy from the surface,

and the electro-chemical dissolving being prolonged until a substantially equalized potential is obtained over the surface of the article.

9. A process according to claim 8, in which the bath is an oxidizing medium.

10. A process according to claim 8, in which the bath is a solution of chromic acid.

11. A process according to claim 8, in which the bath is a solution containing 15% or more chromic acid.

12. A process according to claim 8, in which the bath is a solution containing 15% or more chromic acid and has a temperature of 25 C. or above.

the article being connected in a bath as an anode,

13. A process according to claim 8, in which the article treated is a strained metal or alloy.

COLIN G. FINK. FREDERIC J. KENNY. 

